In a small basement in the Bronx, the pride of place does not go to a shelf occupied by blue models of deformed skulls. Instead, the focus of the Lehman College 3D Virtual and Solid Visualization Laboratory is a large gray printer.

This is no dot-matrix monster. It's a precision three-dimensional printer that lays down layers of liquid blue plastic to create 3-D models of "monkey" skulls (fossil primate skulls, actually). Using this method, the lab's founder, paleoanthropologist Eric Delson, creates detailed replicas of fossil finds as well as larger models of them for closer study. These models serve as teaching tools, allowing students to examine replicas of fossils too rare and fragile for direct contact. As a research aid, 3-D printing simplifies fossil reconstruction and even allows Delson to simulate the skulls of primate ancestors whose remains never fossilized.

Designers in other fields use this technology to print out prototypes to decide on the best possible pattern to send into production. Other researchers have printed working transistors and solar cells. Even printed organs for human beings may soon be viable.

3-D printing has advanced quickly. Only 10 years ago products printed on expensive 3-D printers "looked terrible," Delson says, because the vertical layers were too thick and far apart, producing bubbly surfaces. But five years later the results were so improved that Delson could buy his Objet Eden260 for $150,000. Today, smaller printers now cost tens of thousands of dollars, and home printers like the MakerBot Thing-O-Matic cost less than $1,300.

At the Lehman visualization lab, a window on the 1.2-meter-tall printer reveals a printhead as wired as Medusa's pate next to a central black tray. A digital representation of the tray fills one of two computer screens connected to the printer. At the console, graduate student Claudia Astorino loads a digital image of a mandible; about a minute later, a 3-D lower jaw appears on the digital tray.

Astorino can slide the jaw on screen to a different position, rotate it, even shrink it to the size of a fingernail or blow it up to the dimensions of a modern human chomper. Each of these modifications can be made to all three dimensions, or just one. For example, one skull was doubled in size, width and height, but not in depth, yielding a model that looks like it ran full-tilt into a brick wall, squashing its face toward the back of its head. The misshapen pate adorns the shelf of deformed skulls, along with melted-looking specimens that set improperly or whose eye sockets filled with plastic.

And more than one sample can be printed at once—the jaw on the screen only fills a corner of the tray, leaving room for others. With the click of a mouse, another mandible could join it, or perhaps a model of a full skull. When Astorino sets a print job in motion, she first places as many models as she needs, decides on the proper size, and decides on the best orientations to give the models high resolution without taking too long to print.

Because of the 3-D printing method, the vertical direction will have the highest resolution—but it also takes the longest time to print. When the computer triggers the machine, the printhead moves into action, gliding back and forth over the physical tray and depositing liquid blue plastic to form the very bottom layer of whatever 3-D shape glows on the computer screen. This layer, only about 20 microns thick—about a fifth the width of a human hair—then sets, with the help of ultraviolet light that triggers the transition from liquid to solid. The tray sinks a fraction of an inch, lowering itself gradually into the body of the printer, and then the printing head begins moving again to deposit the next layer. All told, these many layers give the vertical direction a resolution of 1,200 dots per inch (dpi), which is better than the resolution of about 300 to 600 dpi in the other directions.

Although one could optimize the resolution by choosing a position that had the most verticality, this would make the printing take longer. Not only must the printer wait for each layer to set, but the tray must slowly adjust its vertical position before each layer can be deposited. A life-size replica of an australopith skull, for example, might take two days to fully print.

Another complication derives from the fact that the printer lays down layer on layer, bottom first. It cannot print the final product whole, but must prevent parts of it from collapsing with "support material." For example, if one wanted to print an upside-down cup, the print head would first lay down the round sides of a hollow cylinder. When the printer went to lay down the layers that would be the cup's base, without support material they would fall into the center of the cylinder, ruining the model. But if the printer fills the hollow center of the cup with yellowish, semi-transparent support plastic, then the base would print intact.

Depending on the support material created, there are different methods to separate it after printing is complete. Some companies produce support material that dissolves in water, but Delson's lab uses a crumbly material that can be knocked off of the modeling plastic by a blast of water. A water jet, housed in a large plastic box with a clear front, serves this purpose. Operators open a port on the box to pop in a printed model, and then stick their hands through the rubber-gloved portals into the box to pick it up and grab the jet's nozzle. Ready, aim, spray: the support material flies off as the model is turned to expose every surface, and within a couple minutes blue plastic is all that remains. To give the surface a final smoothing and cleaning, the model sits in a bath of sodium hydroxide for about 15 minutes.

Other 3-D printers simply lay down layers of powder, then add material to make the particles stick together. Some even create each layer by cutting it out of standard printer paper. Even metal can replace ink in a 3-D printer. As printing becomes an option for the manufacture of more and more objects, the technology may become even more ubiquitous—even if you're not in the market for models of monkey skulls.

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